Uranium oxide is an important compound for use in the nuclear fuel industry and can be obtained by hydrometallurgical processes from uranium ores.
Uranium-containing ores whose acid-consuming constituents are present in unduly high proportions so that acid leaching is uneconomical, can be leached with alkaline and preferably aqueous soda (sodium carbonate) solutions so that the uranium as dissolved form is found in the uranyl carbonate ion [UO.sub.2 (CO.sub.3).sub.3 ].sup.4-. This ion can be considered to be present as the dissolved form of sodium uranyl carbonate having the formula Na.sub.4 [UO.sub.2 (CO.sub.3).sub.3 ].
Such solutions thus can be considered to be starting solutions for the recovery of uranium which can proceed through precipitation of the uranium oxide UO.sub.2.
Several processes are known for the precipitation of uranium from such solutions, either as the uranium dioxide or in other forms which can ultimately yield the uranium dioxide or a component readily converted to the uranium dioxide.
For example, uranium can be precipitated by the addition of sodium hydroxide to form sodium diuranate Na.sub.2 U.sub.2 O.sub.7. This process can proceed in accordance with the formula 2Na.sub.4 [UO.sub.2 (CO.sub.3).sub.3 ]+6NaOH.fwdarw.Na.sub.2 U.sub.2 O.sub.7 .dwnarw.+6Na.sub.2 CO.sub.3 +3H.sub.2 O.
One of the problems with this method is that the precipitation of the sodium diuranate takes a long time. Another problem is that the sodium hydroxide which is used is comparatively expensive.
The sodium diuranate is then generally introduced into sulfuric acid and the diuranate can be precipitated as the ammonium salt by neutralization with ammonia thereby eliminating the sodium.
In yet another process, alkaline solutions containing low concentrations of uranium can be reduced at temperatures of 100.degree. to 200.degree. and at superatmospheric pressures with hydrogen and hydrogenation or reduction catalysts, e.g. nickel-based catalysts, in accordance with the relationship ##STR1##
This process requires extremely high pressures, the use of expensive hydrogen, costly catalysts and, at least in part because of the high pressures, also expensive equipment.